When a reluctance type motor is used as a driving source for an electric motor car, a regenerative braking is generally required. Although a brushless DC motor be used for the same purpose, the application by a conventional method would be too costly because of the complexity of the composition.
As the reluctance type motor has no magnet rotor, it has been considered that the regenerative braking by power generation is impossible, no relevant prior art has been established. The same can be said as to DC motors too.
The applicant of the present invention has already proposed Unexamined Japanese Patent Application No. 1-31433 and Unexamined Japanese Patent Application No. 1-188367, although the technologies disclosed in these applications differ from each other.
In the case of a reluctance type motor, an extraordinary large inductance of the exciting coil will cause a slow building-up of exciting current at an initial stage of the current supply period and a slow trailing-edge at a terminating stage of the current supply period. The former will cause a drop of output torque, and the latter occurrence of a counter torque.
If the voltage of electric power source is raised order to make building-up of armature current sharp in the initial stage of the current supply period, this will cause the current to increase acutely in a region above the magnetic saturation point, which entails vibrations and electric noises in the motor. Since the above-described building-up section of the armature current corresponds to a section where the torque is small, it a problem that only disadvantages will be amplified.
Thus, it has been a problem that a high-speed rotation (i.e. several tens of thousands rpm) cannot be realized due to above-described torque reduction and counter torque. Even if the rotational speed is reduced down to a generally utilized speed region (i.e. several thousands rpm), still there is a problem that the torque reduction and the counter torque will remain unremoved to cause the fall of the motor's operating efficiency. If a means to be adopted for increasing the output torque is to raise the voltage of the electric power source, this requires the voltage to be raised to a level such as 1000 volts or more, which is unpractical.
Brushless DC motor will also be encountered with the similar problems in the case of a motor generating a large output.
As the reluctance type motor has a rotor with no magnet, no power generation is expected during rotation. Accordingly, obtaining electromagnetic braking is impossible. That is, neither the electromagnetic function nor regenerative braking function are available
Thus, it has been a problem for the reluctance type motors that they have not been able to be readily applicable to the drive sources of servomotors or electric-powered vehicles.
As the exciting coil has a large inductance, the activation/deactivation of the exciting coil is controlled using two switching elements to be respectively inserted at respective ends of the exciting coil. The problem of these switching elements is that they are expensive, since they are required to have a large capacity in order to control large current.
The reluctance type motor has a rotor with a larger number of salient poles and a larger capacity, so that not only a larger amount of magnetic energy is stored into and discharged from the magnetic poles and salient, poles but the magnetic energy is stored and discharged at a greater frequency during each rotation of the motor. It would be, therefore, a problem of the reluctance type motor that its rotational speed is relatively low despite its advantage of being capable of producing a large output torque. The brushless DC motors will be encountered with the similar problems in the case of a motor generating a large output.
Thus, the present invention has an object to provide a reluctance-type motor and a brushless DC motor having high torque and high efficiency in a high-speed region.